(1) Field of the Invention
The present invention relates to a solar cell which converts light energy into electric energy and, in particular, relates to a solar cell with an integrated bypass function in which the function of a bypass diode for protecting the solar cell from a reverse bias voltage is added.
(2) Description of the Prior Art
A solar cell module is typically composed of a large number of solar cells connected in parallel and/or in series in order to obtain a desired output voltage with a desired output current. If part of a solar cell or some of the cells are shaded, voltages generated by the other cells are applied to the shaded cells as reverse bias voltages. If a reverse bias voltage exceeding the peak inverse voltage of a shaded cell (breakdown voltage) is applied to the cell, the cell may be short-circuited and damaged, whereby the output characteristics of the entire solar cell module could be degraded.
In the case of solar cell modules for space applications such as a satellite, part of the satellite body or elements thereof such as antennas, etc. could shade the solar cell module while the attitude of the satellite is controlled. For terrestrial uses, neighboring buildings, for example, can shade the solar cells, or birds flying by the solar cells can deposit fecal matter thereon which shades the solar cells.
As an example of such cases, discussion will be made of a case where a part of the surface of a solar cell module consisting of a number of solar cells connected in parallel is shaded. In a shunt mode as shown in FIG. 4A where a solar cell module M is substantially short-circuited between both terminals thereof, when a submodule 11 is shaded, a reverse bias V.sub.12 generated across the submodule group 12 which remains unshaded is applied to the submodule 11. If the voltage across the submodule 11 is v.sub.11, it can be expressed as V.sub.11 =-V.sub.12.
When an external power source V.sub.B is connected to the solar cell module M as illustrated in FIG. 4B, V.sub.11 is represented by V.sub.11 =V.sub.B -V.sub.12.
That is, when a positive voltage is applied to the N-electrode of the shaded submodule 11, if the reverse bias voltage is greater than the peak inverse voltage of the solar cells constituting the submodule 11, the solar cells are short-circuited and may be damaged, whereby the shaded submodule 11 and consequently the whole solar cell module M may become degraded in output characteristics.
In order to avoid hazards of this kind due to the reverse bias voltage, a bypass diode is provided for each solar cell or for every solar cell module; alternatively, so-called "diode integrated" solar cells are used in which bypass diodes are integrated in the solar cells. Beside these, there also is known a "solar cell with integrated bypass function".
An example of a prior art solar cell with integrated bypass function will now be explained with reference to drawings. FIG. 1 is a plan view showing a structure of a solar cell with integrated bypass function proposed by the present applicant (Japanese Utility Model Application Hei 3-102,749) and FIG. 2 is a sectional view thereof taken on a line 20-21 in FIG. 1.
As illustrated in FIG. 1, a light receiving face of the solar cell is covered with a transparent antireflection coating 8. Under the coating 8, strip shaped N-contact electrodes 7 connected together at their ends with a bar-shaped N-contact electrode connecting portion 5 are arranged like a comb over an N-type region 2.
As shown in FIG. 2, the cell is composed of a P-type silicon substrate 1, the N-type region 2 formed on the light receiving face of the substrate 1, a P.sup.+ -type region 3 formed beneath the substrate 1 for providing a BSF effect, P.sup.+ -type islands 4 partially formed on the light receiving face of substrate 1, the N-electrode 7 formed on the surface of the N-type region 2, the antireflection coating 8 covering approximately the entire N-type region 2 and a P-electrode 6 covering approximately the entire underside of the P.sup.+ -type region 3.
The solar cell thus configured is produced by the procedures shown in step-order sectional views shown in FIGS. 3A through 3F.
First, the whole surface of a P-type silicon substrate 1 shown in FIG. 3A is subjected to thermal oxidation or the like so that an oxide film 10 is formed, as shown in FIG. 3B. Subsequently, as shown in FIG. 3C, the oxide film 10 on the underside is removed, and holes 14, 14, . . . are formed on the surface of the oxide film 10 by the photolithographic or any other technique. These holes 14, 14, . . . correspond to P.sup.+ -type islands 4, 4, . . . which will be formed next. Then, a P.sup.+ -type impurity is diffused into the wafer to an impurity concentration of 1.times.10.sup.20 cm.sup.-3, for instance.
Thereafter, the remaining oxide film 10 on the top and side faces is removed so as to produce a wafer shown in FIG. 3D. This wafer has a number of P.sup.+ -type islands 4, 4, . . . formed on the top surface thereof and the P.sup.+ -type region 3 formed over the undersurface thereof. Subsequently, as shown in FIG. 3E, the N-type region 2 is formed on the top and side faces by the thermal diffusion technique or the like. Since the P.sup.+ -type islands 4, 4, . . . are protected by the boron glass remaining on the topmost layer thereof, these regions are unaffected and kept in the form of islands in the N-type region 2.
Next, as shown in FIG. 3F, the comb-shaped N-contact electrodes 7 and the N-contact electrode connection portion 5 (not shown in the figure) are formed on the top surface. Then, the antireflection coating 8 is formed on the top of the wafer while the P-contact electrode 6 is vacuum evaporated on the undersurface thereof. The thus formed wafer is cut at both ends indicated by broken lines, whereby a solar cell as shown in FIGS. 1 and 2 is obtained.
A large number of the thus prepared solar cells are connected in series and in parallel as shown in FIG. 4A to output a desired voltage and a desired current. The thus formed assembly is used as a typical solar cell module M.
External attachment of bypass diodes to the solar cell for the purpose of preventing the solar cell from being damaged by the reverse bias voltage, results in increased cost for bypass diodes as well as the manufacturing cost of the attachment process. Further, this method suffers from another problem that the packaging density of solar cells is decreased.
Since, in the conventional diode integrated solar cell, a bypass diode and a solar cell are formed together in a silicon substrate, the manufacturing process becomes complicated, resulting in increased production cost. Further, the conventional solar cell with integrated bypass function has island shaped P.sup.+ regions that occupy part of the cell surface, so that the effective area of the solar cell is decreased, whereby the conversion efficiency is lowered.